First off, are you confident enough to build your own computer? If not, then I suggest either CyberpowerPC.com, or Ibuypower.com
Here is a build that may be good for your needs:
iBUYPOWER Computer :: Gamer Mage D285
Here is what you should Customize it to:
Case: Anything you like
Processor: [= Six Core =] AMD Phenom™ II X6 1100T Black Edition Six-Core CPU
Cooling: Choose between liquid cooling or the heatsink. (I suggest the heatsink for non-advanced owners)
Memory: 8 GB [4 GB X2] DDR3-1600 Memory Module
Video Card: AMAZING card that it comes with already, upgrade it if you desire. If willing to upgrade, I suggest the GTX 570
MotherBoard: Keep it at what it is... unless you are eventually going to add another graphics card.
Power Supply: 800 Watt -- Standard
Hard Drive: Keep it at what it is.
Accesories: Choose what you desire.
This computer would murder any game you throw at it, and it would end up at a total of $1299 with shipping.
(Note: They build the computer for you)